Peripheral Nerve Stimulators for Nerve Blocks
What are they and how do they work?

The use of peripheral nerve stimulators (PNS) in regional anesthesia is not new. As early as 1912, Von Perthes described the use of PNS[1]. While the experimentation in this field had been present ever since, the wider use of PNS has awaited the resurgence of interest in regional anesthesia, which occurred in the last 2 decades. With a resurgence of interest in regional anesthesia and demand for more accurate nerve localization before injection of local anesthetics, there has been a corresponding response from the industry, resulting in over a dozen of various units being present on the market. Thus, it comes as a no surprise that we are often asked which nerve stimulator would we recommend. While we would prefer not to suggest a particular model over another, at the end of this discussion you should have enough information to make an informed decision on which nerve stimulator would work well in your practice.

There are at least a dozen of various models of nerve stimulators that can be used in regional anesthesia. Which one should you choose?		A remote control of the current using a foot-controller allows a single-operator block placement.

In 1850 von Helmohlz [2] had shown in a classic series of experiments with an isolated nerve muscle preparation the temporal nature of nerve fiber conduction. His experiments led to for further elucidation of the relevant physiology of peripheral nerve stimulation. Of particular importance is the relationship between the strength and the duration of the current and the polarity of the stimulus. In order to propagate a nerve impulse, a certain threshold stimulus must be applied to the nerve. Of note, below this threshold, no impulse is propagated, and increase in the intensity of the stimulus above this threshold propagation or triggered impulse is further increased. Assuming a square pulse of the current is used to stimulate the nerve, the total energy (charge) applied to the nerve is a product of the intensity of the current and the duration of the of the pulse (sample output). There are two terms important to understanding the nerve stimulation: the reobase and chronaxie. The reobase is the minimal current required to the nerve with a long pulse.

A typical output of a nerve stimulator. For this test, the nerve stimulator was set to deliver 1mA into a 1 kOhm load. The x-axis displays the duration of the stimulus (in this example 100 microseconds) and the y-axis displays the amplitude of the voltage output (in this case 1.15 Volts). The actual measured output is calculated as Voltage output (V)/Impedance (Ohm) = Output (mA)

The chronaxie is the duration of the stimulus required to stimulate at twice the reobase. From the formula I=Ir (1+C/t) where I is the current required, Ir is the rheobase, C is the chronaxie, and t is the duration of stimulus, it is evident that the current needed to stimulate the nerve will depend on the pulse width or duration of the stimulus.

The chronaxie can be used as a measure of the threshold for any particular nerve and it is useful when comparing different nerves or nerve fiber types. Values for chronaxies peripheral nerves are shown in the table [3]. (A better references are needed regarding nerves stimulation).

Looking at the table, it is apparent that the large fibers are more readily stimulated than the smaller fibers. This serves it is possible to stimulate the large a Alpha motor fibers without stimulating the smaller D Delta or C fibers responsible for pain.

Chronaxies of peripheral nerves A - Alpha fibers 50-100 µsec D - Delta fibers 170 µsec C - C fibers 400 µsec

An important principle of peripheral nerve stimulation is the preferential cathodal stimulation [3]. For instance, when the nerve is stimulated by an electrode, significantly less current is needed to obtained a response to nerve stimulation when the cathode (negative) is adjacent to the nerve, rather then the anode (positive is adjacent to the nerve). The reason for this phenomenon is because when the stimulating electrode is negative, the current flow alters the resting membrane potential adjacent to the needle, producing an area of depolarization which then spreads across the nerve. When the electrode adjacent to the nerve is an anode, the current causes an are hyperpolarization adjacent to the needle and a ring of depolarization distal to the needle tip. This arrangement is less efficient in propagating the stimulus and has clinical implication.

Another fundamental issue is the variation of the stimulus intensity (current) with varying distance from the nerve. As the stimulating tip moves away from the nerve, the relationship between the stimulus intensity and the distance from the nerve is governed by Coulombs law: E=K (Q/r²) where E is the current required, K a constant, Q the minimal current, and r the distance. The presence of the inverse square means that a very high stimulus is needed once the tip is some distance from the nerve. This principle is used to estimate needle-nerve distance by employing a stimulus of known intensity and pulse duration [3].

Galindo, Ford, Pither and Raj have recommended desirable features of peripheral nerve stimulators [3,4,5]. While these recommendations are buy and large still valid today, many specialized nerve stimulators are now commercially available. Most of these units contain complex and sophisticated electronics. While the advances in technology largely serve the purpose of obtaining a more reliable and clinically useful nerve localization, the plethora of various functions and features frequently leave clinicians with very little understanding of the basics principles behind nerve stimulation and how nerve stimulators accomplish this task. Based on interaction of over a thousand of anesthesiologists who attended our workshops on peripheral nerve blocks and our recent survey, we feel that an average anesthesiologist has lost a pace with the technology in this field [6].

Constant current output. The impedance of tissues, needles, connecting wires and grounding electrodes may vary (e.g., 1 kOhm-20 kOhm). The constant current design of the stimulator allows for an automatic compensation for changes in tissue or connection impedance during nerve stimulation, assuring accurate delivery of the specified current.

Current meter. The ability to read the current being delivered is of utmost importance, since the current intensity at which the nerve is stimulated gives the operator an approximation of the needle-nerve distance; 0.5 mA or less indicating intimate needle-nerve relationship. Make sure that your stimulator reads in 0.01 mA!

Current intensity control. This can be either via digital means or better an analog dial. Alternatively, the current can be control via a foot controller, allowing a single-operator performance of the nerve block [7] (Video 1). We routinely use a foot-controlled nerve stimulator in our practice as this allows much better control, faster administration of nerve blocks and eliminates the need for a helper person to manipulate the current.

Short pulse width. A short pulse width, e.g. 50 µsec -100 µsec corresponds to chronaxies of Alpha fibers, triggering muscle twitches rather than causing pain on stimulation. Stimulating frequency. Traditionally, PNS have had 1 Hz stimulation, meaning 1 pulse per second. However, two Hz stimulation capability is clinically advantageous, since it allows faster manipulation of the needle.

Disconnect indicator. This is a necessary feature, since the operator should know when the stimulus is not being delivered due to whatever cause (e.g.., disconnect, poor electrical connection, battery failure, etc.).

The nerve stimulator should be best specifically made for peripheral nerve blocks.

Controlling the output current by pressing on the foot pedal substantially facilitates the performance of peripheral nerve blocks.

Because the knowledge of how nerve stimulators work will undoubtedly assist in nerve stimulation and interpretation of responses to nerve stimulation, in the section below we describe basic fundamental principles on which the circuitry of PNS is based.

This circuit has four main parts:

• Oscillator
• Display
• Constant current generator
• Controls

Oscillator

Oscillator is the basis of the nerve stimulator. Its function is to produce a pulse at required frequency and width [8]. This oscillator is based on Atmel 89C52 microcontroller (Data sheet reference, Name and address of manufacturer). This microcontroller is an 8-bit processor with 8Kb of flash program memory and 256 bytes of user RAM. Instructions are stored inside the program memory and are executed when the circuit is powered up. Oscillator is interrupt driven by internal timers of microcontroller. Two timers have been used, one for control of the frequency and one for control of the pulse width. Each frequency (and pulse width) has a certain value that has to be loaded into the timers and those values have been loaded as a table in the memory. Timer 0 controls pulse width. Upon selection of appropriate frequency/pulse width, oscillator output pin goes high, Timer 0 interrupt is set and Timer 0 starts counting up with every clock cycle (90.42 ns). Once appropriate value has been reached, it issues an interrupt to the processor, which in turn clears and disables Timer 0 interrupt and sets the oscillator output low. In the meantime, Timer 1 (controls frequency) also counts up with every clock cycle (90.42 ns). Once it reaches value assigned to the selected frequency it issues an interrupt. This interrupt is then cleared by the processor, which also enables the Timer 0 interrupt and sets the oscillator high. Process is repeated, producing the requested pulse train. Due to its nature, this oscillator is very accurate. It can be controlled within 1 clock cycle of the target frequency with addition of some time for software overhead.

Display

Display is a standard Liquid Crystal Display (LCD). An example used in this circuit design is LCD (Tracer III, Life-Tech). The layout of the display is shown on the figure.

Frequency of the current signal is given in Hz. It can be changed from 1 to 10 Hz. Pulse width is displayed in either milliseconds or microseconds. It depends on what the most appropriate form for display of the current pulse width is. Current is always displayed in milliamperes.

Constant Current Generator

Every pulse, microcontroller outputs a 12-bit number corresponding to the value of selected current. This number is then digital converted to an analog voltage by a 12-bit Digital to Analog Converter (DAC). The purpose of DAC converters is essentially to transcribe the digital data/outputs/whatever into analog ones. This voltage is then used as a reference value for the constant current generator.

Constant current output. The impedance of tissues, needles, connecting wires and groun-ding electrodes may vary (e.g., 1 kOhm-20 kOhm). The constant current design of the stimulator allows for an automatic compensation for changes in tissue or connection impedance during nerve stimulation, assuring accurate delivery of the specified current.

Current meter. The ability to read the current being delivered is of utmost importance, since the current intensity at which the nerve is stimulated gives the operator an approximation of the needle-nerve distance; 0.5 mA or less indicating intimate needle-nerve relationship. Make sure that your stimulator reads in 0.01 mA!

Controlling the output current by pressing on the foot pedal substantially facilitates the performance of peripheral nerve blocks.

Current intensity control. This can be either via digital means or better an analog dial. Alternatively, the current can be control via a foot controller, allowing a single-operator performance of the nerve block[1]. We routinely use a foot-controlled nerve stimulator in our practice as this allows much better control, faster administration of nerve blocks and eliminates the need for a helper person to manipulate the current.

Short pulse width. A short pulse width, e.g. 50 µsec -100 µsec corresponds to chronaxies of a Alpha fibers, triggering muscle twitches rather than causing pain on stimulation.

Stimulating frequency. Traditionally, PNS have had 1 Hz stimulation, meaning 1 pulse per second. However, make sure that your stimulator has a 2 Hz stimulation capability. This is clinically more advantageous, since it allows faster manipulation of the needle.

Disconnect indicator. This is a necessary feature, since the operator should know when the stimulus is not being delivered due to whatever cause (e.g., disconnect, poor electrical connection, battery failure, etc.).

Make sure that your nerve stimulator is specifically made for peripheral nerve blocks and it is not one of those can-do-everything units. In other words, we prefer the stimulators that can do only low-output stimulation and are not capable of monitoring neuromuscular blockade. This feature just makes things much more confusing.

All of the recommended have most of the above features and we have used them all with great success in our practice.

An excellent new unit, meeting all of the above criteria. Also, this is the only unit which has the foot pedal current control capability. Manufacturer: LifeTech, Inc		An overall excellent unit. Very light-weight, accurate and portable. Probably one of the most widely used units worldwide. No foot-control capability. Manufacturer: B.Braun, Inc.

An excellent new unit, meeting all of the above criteria. No foot pedal current control capability. Manufacturer: B.Braun, Inc.		An excellent and very precise peripheral nerve stimulator made in Germany by Pajunk and distributed in US by PNA Medical Systems.		A very nice newer unit, meeting most of the above criteria. Output current measured in 0.1 mA increments. No foot pedal current control capability. Manufacturer: HDC, Inc.

1. 1. Perthes VG. Uker leitungsanasthesie unter zuhilfenahme elektrischer reizung. Munchener Medizinische Wochenschrift 1912:47:2545-48.
2. 2. Von Helmholtz H. Messungem uber den zetlichen verlaug der Zuchung animalischer Muskern und die Fortphan zungsgeschwindigkeit der Reizung in den Nerven. Archive Anatomie und Phisiologie 1850:277.
3. 3. Pither CE, Raj PP, Ford DJ. The use of peripheral nerve stimulators for regional anesthesia: A review of experimental characteristics. Regional Anesthesia 1985:10:49-58. 4. Galindo A. Electrical localization of peripheral nerves: instrumentation and clinical experience. Regional Anesthesia 1983:8:49-50.
4. 5. Ford DJ, Pither CE, Raj PP. Electrical characteristics of peripheral nerve stimulators: implications for neve localization. Regional Anesthesia 1984;9:73-77.
5. 6. Vloka JD, Hadzic A, Kuroda MM, Koorn R, Birnbach DJ. Practice patterns in the use of peripheral nerve stimulators in peripheral nerve blockade. A national survey. Regional Anesthesia, 1997;22 (2S):61.
6. 7. Hadzic A, Vloka JD. Peripheral Nerve Stimulator for Unassisted Nerve Blockade. Anesthesiology, 1996; 84(6): 1528-1529.
7. 8. Brey, B.B. Basic I/O Interface. In: Brey, B.B.: The Intel Microprocessors 8086/8088, 80186/80188, 80286, 80386, 80486, Pentium, and Pentium Pro Processor: Architecture, Programming, and Interfacing, Fourth Edition, Prentice Hall, Upper Saddle River, NJ, 1997, pp 362-429.